It is well known that cycloaliphatic polyisocyanates can be used for the preparation of aqueous polymer dispersions. Generally, the dispersions are prepared by a two-stage process wherein an isocyanate-terminated polyurethane prepolymer is formed in the first stage by reacting an excess of polyisocyanate with an isocyanate reactive compound. The prepolymer is then dispersed in water and the remaining isocyanate is further reacted to form an aqueous polyurethane dispersion. Cycloaliphatic polyisocyanates have been shown to provide polyurethane-urea polymers having increased water resistance and light stability compared to similar polymers based on aromatic polyisocyanates. The cycloaliphatic polyisocyanates most often used for the preparation of aqueous polyurethane-urea dispersions include isophorone diisocyanate (IPDI.RTM.), hydrogenated diphenyl-methane diisocyanate (H.sub.12 MDI) and hydrogenated meta-xylene diisocyanate (H.sub.6 m-XDI) which are further described in U.S. Pat. No. 4,147,679 (Scriven et al.), U.S. Pat. 4,870,129 (Henning et al.), U.S. Pat. No. 4,742,095 (Markusch et al.), and U.S. Pat. No. 5,608,000 (Duan et al.).
Recent literature disclosed norbornane diisocyanate (NBDI) as a new class of isocyanate. For example, Product Data information from Mitsui Toatsu Chemicals, Inc., June, 1995 generally describes the characteristics of NBDI and polymers formed therefrom. Examples given describe NBDI as a raw material for the production of non-yellowing polyurethane, polyurea, or polyisocyanurate resins in the field of paints and coatings. The literature also states, "as NBDI has a stiff Norbornane structure, it would be expected that the polymers based on NBDI show higher thermal stability and hardness than ones based on isocyanates with a linear chain structure such as HDI.". The reference did not teach or suggest the use of NBDI as a raw material for the preparation of aqueous polymer adhesives and the polymer properties therefrom.
Therefore, there remains a need for aqueous polymer adhesives which provide enhanced bond strength when subjected to increased thermal and mechanical stress.